The invention generally relates to semiconductor devices and methods for fabricating such devices. More particularly, the invention relates to a power amplifier, and more particularly, to a power amplifier that controls a bias current of an output terminal.
Power amplifiers are often capable of manipulating a voice bandwidth and, therefore, are used in portable electronic devices such as cellular phones. These power amplifiers must be able to both inhibit the generation of distortion, as well as use a minimum amount of power.
Power amplifiers contain many components, including an output terminal. The structure of the output terminal plays an important role in determining whether (and to what extent) the power amplifier is able to prevent distortion and operate at low levels of power. For example, a conventional class AB output terminal that is able to suppress the generation of distortion and that consumes a low level of power is described and illustrated in FIG. 1.
As shown in FIG. 1, base voltages of NPN bipolar transistor Q1 and of PNP bipolar transistor Q2 are biased by diodes D1 and D2, respectively. Accordingly, the NPN and PNP transistors Q1 and Q2 are always switched on. By adjusting a current source IB when there is no signal (a current flowing to the output terminal when there is no signal is referred to as a Q current), a bias current applied to the NPN and PNP transistors Q1 and Q2 can be varied.
In an ideal circuit, the output terminal should not generate any distortion. However, in an actual circuit, the base voltages are determined so that a suitable current flows to the NPN and PNP transistors Q1 and Q2. Thus, a certain amount of time is required to alter the base voltages to avoid an abrupt flow of current or if no current is to flow at all. This time delay causes the generation of crossover distortion.
Crossover distortion is particularly severe when there is a large load or when a signal of a high frequency is applied. When there is a large load, the current that must be supplied to the load is extremely large. Thus, distortion increases because of the inability of the output transistor to abruptly supply current. Where a signal of a high frequency is applied, a push-pull operation of the output transistor must occur very quickly. However, because of the delay that typically occurs, crossover distortion increases.
To prevent such time delay, it is necessary to increase the bias current (Q current) so that a large current flows to the output terminal when there is no signal, thereby decreasing the time required to obtain a desired current. While increasing the bias current to the output terminal is effective in preventing the occurrence of distortion, a large amount of power is unfortunately consumed as a result.
The invention provides a power amplifier that simultaneously uses a minimum amount of power while preventing the generation of distortion. The power amplifier increases a bias current of an output terminal when a crossover distortion is severe, and decreases the bias current of the output terminal when there is no signal and no distortion.
In one aspect, the invention provides a power amplifier including: a differential input terminal for receiving a first voltage and a second voltage respectively through a first terminal and a second terminal, and for outputting a current corresponding to a difference in the input voltages; an output terminal for outputting an amplified signal corresponding to the current output by the differential input terminal; a negative feedback circuit for performing negative feedback of an output voltage of the output terminal and for providing the output voltage to the first terminal of the differential input terminal; and an adaptive bias current control circuit for controlling a bias current of the output terminal according to distortion information corresponding to the difference between the first voltage and the second voltage.
In one aspect of the invention, the output terminal can comprise: an output transistor unit including first and second transistors that supply a current to a load; first and second variable current sources controlled by an adaptive bias current control circuit; first and second current sources provided by the differential input terminal; a first control transistor unit that responds to the first variable current source and the first current source, and that controls an input voltage of the first transistor; and a second control transistor unit that responds to the second variable current source and the second current source, and that controls the second transistor, where the first and second control transistor units control a bias current flowing to the output transistor unit when there is no signal.
In one aspect of the invention, the differential input terminal comprises: a third transistor and a fourth transistor, the third transistor including a gate to which a first voltage is applied and which outputs a current corresponding to the first voltage, and the fourth transistor including a gate to which the second voltage is applied and which outputs a current corresponding to the second voltage; a third current source connected to the sources of the third and fourth transistors; a fifth transistor for copying a drain current of the third transistor and supplying to the output terminal a current corresponding to a current output by the second current source; and a sixth transistor for copying a drain current of the fourth transistor and supplying to the output terminal a current corresponding to a current output by the first current source.
In one aspect of the invention, the adaptive bias current control circuit can comprise: a distortion sensor for obtaining and outputting distortion information from a difference between the second voltage input to the second terminal of the differential input terminal and the first voltage that is input with the feedback of the output voltage of the output terminal; an absolute value circuit for outputting an absolute value of the distortion information output by the distortion sensor; and a distortion signal amplifier for receiving the absolute value of the distortion information output by the absolute value circuit and for outputting a control signal for controlling the first and second variable current sources.